One of the developers I correspond with has been working on some new backends for nano-x/microwindows. He's added an allegro back end and he's also done some work on SDL support. Code is at https://github.com/ghaerr/microwindows

I've been able to get nano-x to build (with server builtin instead of separate) using the SDL 1.2.x back end. From there, was able to get FLTK to build with some minor patching. It's not perfect, but it does provide a way to get simple X and FLTK based applications to work without requiring X.